1. Field of the Invention
This invention relates to compositions that change color in response to temperature and humidity. More particularly, the invention relates to compositions that can be used in conjuction with articles to be sterilized to indicate whether these articles have been exposed to sterilizing conditions and whether the sterilization was accomplished by steam or dry heat.
2. Description of the Prior Art, and Other Information
Compositions that change color with changes in temperature have been known since at least 1928 when Davis applied for his patent (issuing as U.S. Pat. No. 1,777,353), which discloses that when the reaction product of cobalt cyanate with a tertiary base is dissolved in a suitable solvent such as chloroform, benzene, alcohol, acetone, toluene, or xylene, the resulting solution has the property of changing color from clear pink to lilac to violet to deep blue with increasing temperature. Such a composition cannot be used as a sterilization indicator, however, since the color change probable reverses with decreasing temperature and thus does not record a pre-existing temperature condition (Davis does not state ipsis verbis that the color change is reversible, however, reversibility is implied in the discussion of equilibrium, and also by the fact that: EQU blue colored sol.+pyridine.fwdarw.pink color EQU pink colored sol..fwdarw.blue sol.;
a complex forms at room temperature but is unstable at elevated temperatures). Even if this defect were not present, such solutions would be of limited usefulness, since generally they boil at no more than about 135.degree. C., a temperature very near ordinary sterilization temperatures (the boiling point depends upon the solvent and the color changing temperature depends upon the solvent and quantity of pyridine in solution). Further, these solutions show only changes in the hue of the deep blue color, and not the color itself, between 100.degree. C. and 135.degree. C., the ordinary temperature range for sterilization.
In 1958, Huyck and Romito (U.S. Pat. No. 2,826,073) mentioned, but did not describe, "thermosensitive indicating materials capable of undergoing color changes" for use in devices for indicating completion of sterilization processes. The following year Korpman described (U.S. Pat. No. 2,889,799) a pressure sensitive adhesive tape for packaging surgical articles which incorporates a composition that undergoes a color change upon exposure to conditions necessary for sterilization. The composition may be one of several specified heat modifiable organic dyes, or organic dye intermediates or derivatives. It may also be a metallic salt system, (a lead oxide and sulfur system being specified), one of certain pH indicator systems, or one of certain diazonium compound systems. These materials have several drawbacks, however. First, organic dyes, and particularly those used as pH indicators, are prone to degrade under the ambient conditions associated with sterilization, such as exposure to actinic radiation. See Emigh, infra, at col. 2, lines 43-50. The alternate use of systems involving heavy metal compounds is undesirable because of the high toxicity of many of these which would prove detrimental in many cases, e.g., where the sterilized articles are for use in medicine. Another drawback to the use of the materials of Korpman is the inability of these materials to indicate the type of sterilization employed, e.g., whether steam or dry heat sterilization is being employed. See col. 3, lines 9-25 of U.S. Pat. No. 2,889,799. Such information is improtant since some sterilization methods should not be used for certain articles. Arthur H. Brian et al, BACTERIOLOGY 79-82 (6th ed. 1972).
In 1972, Emigh disclosed (U.S. Pat. No. 3,684,737) heavy metal halide coating compositions for use in indicating sterilization by ethylene oxide, steam or dry heat. As mentioned above, however, the use of heavy metal salts, many of which are quite toxic, is undesirable in many cases. Another disadvantage of the Emigh composition is that it does not so distinctly differentiate among the types of sterilization employed as to be unambiguous.
In 1974, Chapman disclosed in U.S. Pat. No. 3,837,809 a sterilization indicator using a sulphite, such as sodium sulphite. However, it is useful only for steam sterilizations employing formaldehyde vapor, and has the drawback of producing caustic byproducts such as, in the case of sodium sulphite, sodium hydroxide.
In 1976, Augurt mentioned in U.S. Pat. No. 3,991,881 the simultaneous use of separate indicators for steam and ethylene oxide sterilization, but did not describe the materials used for either.
Of further interest are U.S. Pat. Nos. 3,969,264; 3,568,627; 4,063,878; 3,038,812; 3,430,491; 4,014,260; 3,901,148 and 4,006,686.
U.S. Pat. No. 3,969,264, issued to Davis, discloses the concept of encapsulating liquid crystals which are heat responsive. However, the patent states that the colors are dull because the encapsulating material is not sufficiently transparent, although a color change apparently is perceivable. It does state that another patent, U.S. Pat. No. 3,620,889, calls for the use of a clear plastic resin in combination with liquid crystals. However, in column 1, lines 25-28, the U.S. Pat. No. 3,964,264 states that microencapsulated materials can not be printed.
U.S. Pat. 3,038,812 discloses the use of microencapsulated temperature indicators which are attached to paper by dipping the paper into a slurry, including such microencapsulated materials.
U.S. Pat. No. 3,568,627 discloses a steam sterility indicator which utilizes an indicator bar and a standard bar. There is further provided indicator material which encompasses numerals to show that the entire indicator card has been sterilized. It is uncertain as to whether or not the numerals are actually part of the indicator itself.